1. Field of the Invention
This invention relates to the electrolysis of sodium chloride brines, and in particular to a method in which brine is provided to the anolyte compartment of a bipolar electrolytic cell while water is fed to the catholyte compartment, the anolyte compartment being separated from the catholyte compartment by an ion-exchange membrane which is permeable to sodium ions but substantially impermeable to chlorine ions.
2. Description of the Prior Art
Prior to the present invention, the commercially used equipmemnt for the large-scale electrolysis of brine has been either a mercury cell or a diaphragm cell. Both the mercury cell and the diaphragm cell have particular drawbacks. Although the mercury process yields a caustic of low salt content, it gives considerable difficulty in respect to compliance with pollution-control laws and regulations. The diaphragm process inherently produces a caustic product which contains a proportion of salt, and while there are some purposes for which such a product may be used without difficulty, there are a number of other purposes which require a substantially salt-free product.
Various ion-exchange membrane materials have been available for over twenty years, it has long been apparent to those skilled in the electrolysis of brine that it would be desirable to have a process in which a permselective membrane is used in a cell to separate the anolyte compartment from the catholyte compartment, in place of the asbestos diaphragms customarily used in the diaphragm process. In laboratory tests, cells operating with permselective membranes as separators have been made and tested. There has not, however, been designed a commercially feasible process, because there are many difficulties which must be overcome before the idea of using a permselective membrane as a separator in a chlor-alkali cell can be brought to commercial reality. Some of the early membranes did not have adequate physical strength, and others did not have adequate inertness with respect to the brine and the caustic solutions with which the membrane was required to be in contact for a substantial service life. Still other materials, although satisfactory in regard to the requirements just mentioned, gave disappointing results in respect to the cell voltages required and the current efficiencies observed. Commercial cells for the electrolysis of brine deal with immense quantities of liquids and use immense amounts of electrical power, while producing products (chlorine and caustic) which are relatively inexpensive. A new process which must compete with the existing mercury and diaphragm processes must be relatively low in its consumption of electrical power. A reduction in cell voltage of 0.15 volt, or an increase in current efficiency of two percent, is certainly significant in terms of the costs of practicing the process. The costs are also significantly affected by the frequency with which it is necessary to interrupt the operation of the process. In general, interruptions any more frequent than about once a month are not tolerable. The art has been advanced by the development of dimensionally stable anodes, made of ruthenium- oxide- or platinum-coated titanium or the like, in place of the graphite anodes previously used.
One particular problem, in connection with the operation of membrane-type cells, has been that the cell voltage tends to rise with time, and it has been appreciated that this increase in cell voltage is associated with the precipitation of calcium hydroxide particles within the membrane. This implies that the control of the calcium content of the brine fed to the process is of considerable importance.
Those skilled in the art of operating diaphragm cells are familiar with the practice customarily employed to reduce the calcium content of raw brine to a satisfactorily low level, such as about ten parts per million of calcium. The raw brine is treated with sodium carbonate to precipitate calcium ions as calcium carbonate. The treated brine is freed of calcium carbonate by settling and/or filtration. Some fine particles of calcium carbonate survive this treatment, so that the treated or polished brine ordinarily contains, as mentioned above, about ten parts per million of calcium. The volumes of liquid dealt with are so immense, however, and the times of operation are so long that even this small proportion of calcium is sufficient to give difficulty. Those skilled in the art have not known, prior to the present invention, how this difficulty may be overcome, and a reliable, economical membrane-cell process thus practiced.
In general, there is good reason for those skilled in the art to look away from the improving of the quality of the brine and reducing or interrupting the supply of current to the cell as means to maintain a low cell voltage. Although other things being equal, a purer brine is obviously desirable, it has not been apparent to those skilled in the art that the expense of practicing a better calcium-removal method could be justified, and that the control of calcium, in a membrane electrolysis process, is as important as it is. There has been instead a tendency to try other membrane materials, in the hope of finding one that will work better and not make it necessary to change the existing brine-treatment practices.